Applying a standing-travelling wave decomposition to the persistent ridge-trough over North America during winter 2013/14 Oliver Watt-Meyer Paul Kushner

Applying a standing-travelling wave decomposition to the persistent ridge-trough over North America during winter 2013/14Oliver Watt-MeyerPaul Kushner

Department of PhysicsUniversity of Toronto(currently ASP Graduate Visitor at NCAR)

MODES WorkshopNCAR, Boulder, COAugust 28, 2015

Introduction• 2013/14 winter atmospheric circulation over North

America dominated by a persistent ridge-trough– Led to unusually cold temperatures over Central/Eastern N.A.– Warm and dry conditions on west coast of U.S.

MODES Workshop O. Watt-Meyer 1

Tem

pera

ture

[°C]

Z [m

]

2013/14 NDJFM, 2m Temp anomaly 2013/14 NDJFM, Z500 anomaly

Introduction• In addition, several cold air outbreaks occurred during

the season, the strongest of which was on 7 Jan 2014– Minimum daily temperature records set at many weather

stations, e.g. New York, Chicago, Atlanta [Screen et al., in press]


Tem

pera

ture

[°C]

Z [m

]

CENA (Central/Eastern North America)

7 January 2014, 2m Temp anomaly 7 January 2014, Z500 anomaly

Introduction• Several studies have examined causes of the

seasonally-averaged anomalous circulation pattern [Wang et al., 2014; Hartmann, 2015]




• Screen et al. [in press] focus on the 7 January 2014 event, and show its decreasing likelihood under global warming and Arctic sea ice loss scenarios




• Screen et al. [in press] focus on the 7 January 2014 event, and show its decreasing likelihood under global warming and Arctic sea ice loss scenarios

• I will use a spectral decomposition to distinguish quasi-stationary (i.e. standing) wave variability from synoptic (i.e. travelling) variability over the 2013/14 winter season, and quantify their relative importance for the 7 January 2014 cold air outbreak


Outline1. Connect atmospheric circulation and surface

temperature over North America2. Extremes over last two winter seasons3. Sub-seasonal evolution over 2013/144. Standing-travelling wave decomposition: theory and

application to North American winter circulation5. Conclusions


Data• NCEP-NCAR Reanalysis 1– 1958-2015, daily data

• 2m air temperature– CENA (Central/Eastern North America): 70-100°W, 26-58°N,

following Screen et al. [in press]

• 500hPa geopotential height (Z500)– DCI (Dipole Circulation Index) to be defined shortly

• Focus on extended winter season– NDJFM


Circulation-temperature connection

• Define Dipole Circulation Index (DCI) to represent strength of ridge-trough over North America:

MODES Workshop 6

NDJFM Z500 Climatology

O. Watt-Meyer


• DCI is well correlated with CENA temperature on daily and interannual timescales

• Correlation increases to r=-0.67 if data detrended• Daily correlation (over all NDJFM days) is r=-0.61MODES Workshop 7O. Watt-Meyer

NDJFM-mean timeseries


• Correlations with the DCI:

MODES Workshop 8O. Watt-Meyer

Contours = ±0.1, ±0.2, ±0.3, etc.

Winter-mean extremes of the last 2 years

• The last two winters had the 2nd and 3rd largest NDJFM-mean DCI since 1958/59

• 2013/14 was the coldest winter over the CENA region since 1958/59; 2014/15 the 6th coldest

Dipole Circulation Index CENA temperature


Sub-seasonal evolution for 2013/14

7 January,2014


Climatology


• Zonal eddy of Z500 at 48°N for winter 2013/14

• Large component of “quasi-stationary” variability• This includes time-mean (ω=0)

component and also some slow variability about it

• Superimposed on this background are faster eastward travelling (synoptic) waves

Nodes of DCI


Standing-travelling wave decomposition



• Using 2D discrete Fourier transform, write signal as:



• Using 2D discrete Fourier transform, write signal as:

• Then make a decomposition of into standing and travelling components:

• This is motivated by:


Watt-Meyer and Kushner [2015]


• Graphically:




• Graphically:

• Because standing and travelling waves are not orthogonal, there is no unique decomposition





• Toy example: wave-1, ω=±(1/30days)



• Toy example: wave-1, ω=±(1/30days)

Variance explained• The power spectrum is decomposed as:

Variance of standing wave atwavenumber , frequency

Variance of travelling wave

Covariance of standing and travelling waves


Variance explained• The power spectrum is decomposed as:

Variance of standing wave atwavenumber , frequency

Variance of travelling wave

Covariance of standing and travelling waves

TotalStandingTravellingCovariance

Example:Wave-160°N500hPaNDJFM

Pow

er


Variance explained

• Historical methods do not explicitly account for the covariance between standing and travelling waves [Hayashi 1973, 1977, 1979; Pratt 1976]

• Broadly speaking, our method recovers similar standing wave variance, but less travelling wave variance



Example: wave-1, 60°N, 100hPa, NDJFM 1979/1980

Wave 1 at 60°N



• Lag coherence and phase between wave-1 at 60°N and 500hPa, and wave-1 at 60°N and other vertical levels [e.g. Randel, 1987]

• Westward travelling wave-1… normal mode?

Correlations with DCI


Contours = ±0.1, ±0.2, ±0.3, etc.





7 January,2014

Daily distribution of DCI for 2013/14


• Overall distribution of DCI shifted positive for 2013/14• Extreme large (above 99.9th percentile) eastward

travelling DCI on 7 January 2014

Grey: histogram of DCI over all NDJFM daysRed: histogram of DCI over 2013/14 NDJFM daysVertical black line: value of DCI on 7 January 2014

Conclusions• Novel standing-travelling wave decomposition

properly accounts for covariance between these wave types– It also allows for straightforward reconstruction of real-space

signals

• Last two boreal winters had strong and persistent ridge-trough structure over North America, accompanied by cold temperatures over Central/Eastern North America

• Record cold temperatures on 7 January 2014 driven by extreme high amplitude synoptic wave


For more details on spectral method:O. Watt-Meyer and P. J. Kushner (2015), J. Atmos. Sci., 72, 787-802.

Extra slides

Extra slides

Extra slides

Extra slides

Extra slides

Extra slides

Extra slidesClimatology Total Anomaly Standing Anomaly West Travelling East Travelling


19 February,2015

Daily distribution of DCI for 2014/5

Grey: histogram of DCI over all NDJFM daysRed: histogram of DCI over 2014/15 NDJFM daysVertical black line: value of DCI on 19 February 2015

Documents

Applying a standing-travelling wave decomposition to the persistent ridge-trough over North America during winter 2013/14 Oliver Watt-Meyer Paul Kushner